A backlight source using a light emitting diode (hereinafter, referred to as an “LED”) has been recently widely used since the backlight source is greatly advantageous in terms of being thinner, more lightweight, and having higher efficiency than a light source using a cold cathode tube.
In order to implement a thinner light source, rather than a direct type in which an LED light source is disposed directly under a liquid crystal monitor or display, an edge light type is frequently used in which a light guide plate as a thin acryl plate or a polycarbonate plate having a rear surface on which minute patterns are formed is used, and light is emitted from a front surface of the light guide plate, that is, surface light emission occurs by causing light incident to an edge surface of the light guide plate.
In the edge light type, it is considered important that light radiated from a light source such as an LED chip is incident to the light guide plate, the light is controlled to be distributed in a desired direction, and thus light is uniformly emitted without causing a bright line or unevenness such as non-uniformity on the light guide plate. For this, if light is caused to be uniformly incident to the light guide plate, and a pattern of the light guide plate is appropriately disposed, light can be controlled to be distributed as desired without generating luminance unevenness or a dark line.
FIGS. 9A and 9B illustrate a configuration of a surface light source lighting device of the related art. In the surface light source lighting device of the related art, as illustrated in FIG. 9A, light from LEDs 32 is guided into light guide plate 34 from incidence edge surface 35 via light incidence lens 33, and the light travels to an opposite side while being reflected at planar portion 36 of a rear surface of light guide plate 34 as illustrated in FIG. 9B.
Light guide patterns 37 are formed on planar portion 36. Light guide plate 34 is made of a resin material such as acryl or polycarbonate, and thus causes total reflection at an incidence angle of about 41° or more due to a difference in a refractive index from air. Light reaches light guide pattern 37 in the middle of being guided inside light guide plate 34, the light is reflected toward a front surface of light guide plate 34 from light guide pattern 37, and thus light is emitted from that portion.
Such light guide patterns 37 are formed on the entire rear surface of light guide plate 34 with a specific pitch, and thus reflection occurs toward the front surface of light guide plate 34 at each location so that light is emitted from the entire front surface of light guide plate 34.
Light from LED 32 is generally radially generated. In a case where the light is incident to light guide plate 34 and reaches light guide pattern 37, the light is obliquely reflected toward the front surface of light guide plate 34 from light guide pattern 37 or obliquely reflected toward the rear surface of light guide plate 34 instead of being reflected toward the front surface of light guide plate 34, depending on an incidence angle, and thus light radiated from the surface light source lighting device is spread.
Thus, light incidence lens 33 which is designed to form substantially parallel light is inserted before light guide plate 34, and thus light close to parallel light is incident to light guide plate 34. Light incidence lens 33 is also formed in a plate shape by using a light transmissive material such as acryl or polycarbonate, and a plurality of light incidence lenses are arranged in parallel so as to form light incidence lens array 38. Light radiated from LEDs 32 is transmitted through respective light incidence lenses 33 so as to be converted into substantially parallel light, and is then incident to light guide plate 34 from incidence edge surface 35. Consequently, an amount of light which obliquely reaches light guide patterns 37 is reduced, and thus it is possible to control a direction of light toward the front surface of light guide plate 34.